Blackness. And the ability to spawn anything. First I'd spawn a star. Then a gas Giant. Then some moons... And so on. Eventually I'd... Well, I'd never get bored. And I would wish to be able to grow and shrink. Mabie I would be at war with my friends.. Mabie we are spawning comets and bombarding planets. The trick: A black, desolate, universe.
It's like the ultimae sandbox game, I think it would be fun.